Evaluation of Greenhouse Gas Emissions from Septic ... - Geoflow

total methane production value of 18 g CH 4 /capita·d is calculated (see Appendix D). The Sassemodel also accounts for methane that leaves the tank in the effluent. For purposes of thecalculation, it was assumed that 25% of the methane produced leaves the tank dissolved in theeffluent.2.4.5 Gases in Effluent Dispersal SystemsIn soil-based effluent dispersal systems, wastewater is applied typically using a system ofperforated pipes. At the point where septic tank effluent is applied to the soil, a clogging zoneoccurs as a result of high moisture content and excess organic matter. The clogging zone iscomposed of various materials, including wastewater particulate matter, microbial biomass, andinorganic precipitates. Based on oxygen diffusion rates alone, it has been determined that thesupply of atmospheric oxygen is a limiting factor (Janna, 2007; Erickson and Tyler, 2001).Thus, the development of anaerobic conditions and clogging zones in conventional soil dispersalsystems is an expected phenomenon.2.5 Summary of GHG Emissions from Wastewater SystemsThe increase in carbon dioxide and other gases in the atmosphere have motivated entities,such as the IPCC, to build GHG inventories to determine critical emitting sources. The publishedstudies on GHG emissions from wastewater treatment plants and the relative importance ofseptic tanks gas emissions are summarized in this section.2.5.1 Observations of GHG Emissions from Wastewater Treatment PlantsConcern about climate change has resulted in increased research on the emission ofGHGs to the atmosphere (IPCC, 1996; U.S. EPA, 2006; Sahelli, 2006; Chandran, 2009; Foleyand Lant, 2009). In general, onsite wastewater treatment systems have received less attentioncompared to full-scale wastewater treatment plants when accounting for GHG releases.However, it is important to have an understanding of the GHG estimated in these studies and thedifferent approaches used to obtain them. Sahely (2006) used a life cycle assessmentmethodology to quantify GHG emissions from municipal wastewater treatment facilities inCanada, reporting carbon dioxide as the major gas contributing to GHG emissions, due to thepredominance of aerobic treatment processes. It should be noted that life cycle assessment(LCA) studies are highly influenced by the boundary conditions and individual assumptions;consequently the findings should be considered only as a baseline for emissions inventories.Foley and Lant (2009) published an experimental approach to evaluate gas fluxes fromWWTPs in Australia. The study focused on the estimation of methane and nitrous oxideemissions from four full-scale treatment systems. The researchers pointed out that the estimatedmethane emissions from wastewater collection systems are underestimated and suggested thatmodels should be developed to address this situation. Liquid methane measured at the inlet andoutlet of various WWTPs are summarized in Table 2-10.Cakir and Stenstrom (2005) developed a mass balance model to compare methane andcarbon dioxide gases from aerobic and anaerobic wastewater treatment systems. The aerobictechnology studied was a conventional activated sludge process and the anaerobic technologywas an upflow anaerobic sludge blanket (UASB) reactor. It was reported in the study that aerobicprocesses release less GHG than anaerobic treatment processes for low strength (~300 g/L)influent BOD u (ultimate carbonaceous oxygen demand or 20 d BOD). According to the model,2-24